The present invention relates to a superconducting microstrip filter comprised of superconducting microstrip lines, for example a superconducting microstrip filter preferred when used for a receiver apparatus of a base station in a mobile communication system.
According to the above example, an input stage of a receiver apparatus of a base station requires as one essential component a filter for passing only signals of frequency bands required for communication. In this case, a filter exhibiting so-called steep cut characteristics is needed in order to make it possible to sufficiently accommodate the rapid increase in the number of mobile communications users, that is subscribers, of recent years at the base station. This is because, the steeper the cut characteristics, the more possible it becomes to use predetermined frequency bands to increase the number of accommodated subscribers.
As a filter capable of obtaining such steep cut characteristics, a filter configured by a plurality of resonators that are cascaded in multiple stages is being employed at present. The larger the number of stages of these resonators, the steeper are the cut characteristics.
On the other hand, however, the inconvenience occurs that the larger the number of cascaded stages of the resonators, the larger an insertion loss in the pass band of the filter.
In order to avoid such an inconvenience, usage of a filter comprised of a superconducting material in place of filters comprised of non-superconducting metal which have been conventionally generally used has been proposed in recent years. Research and development have been underway for commercialization of such a filter. This is a superconducting microstrip filter. Since a surface resistance of a superconducting material is smaller than the surface resistance of non-superconducting metal by two to three orders, an extremely low insertion loss can be realized in the pass band while maintaining the steep cut characteristics. The present invention covers such a superconducting microstrip filter. Note that, below, this will also be simply referred to as a superconducting filter.
The base station based on the above example must receive a further higher power at the receiver apparatus along with the increase of the number of subscribers in recent years. Also, this receiver apparatus is connected to a duplex antenna, so inevitably receives wraparound power due to its own strong transmission power. Furthermore, this base station is provided with a few duplex antennas in proximity to each other, so also receives strong transmission power from adjacent channels.
Under such a circumstance, a higher power resistance is required for the filter in the receiver apparatus. Namely, a high enough power the power resistance must be sufficiently high for the cut characteristics of the filter to be maintained without deterioration even if high power applications of the filter are required.
However, there is a deficiency in that the power resistance is remarkably inferior in the case of a superconducting filter in comparison with a general filter made of ordinary metal. This deficiency is derived from a critical temperature (Tc) inherent in the superconducting filter and a critical current density (Je) inherent in the superconducting filter. Among them, particularly the critical current density (Je) has an extremely close relationship with realization of the function of the superconducting filter.
Accordingly, an improvement of the power resistance must be achieved while keeping the current density below the critical current density (Jc). Note that, it is also essential to maintain the temperature below the critical temperature (Tc), but this depends upon the capacity of an external cooling machine, and is not particularly referred to in the present invention.
As will be explained in detail below by using the drawings, as a known superconducting filter improved in the power resistance, for example, the filter disclosed in the document xe2x80x9cHigh-Power HTS Microstrip Filters for Wireless Communicationsxe2x80x9d, Guo-Chun Liang etc., IEEE Trans. On MTT, vol. 43, No. 12, Dec. 1995, is already known. In each resonator comprising this filter, the line width is enlarged by reducing the characteristic impedance of the line and concentration of current is suppressed. Specifically this is a filter wherein the line width over the entire length of the lines of the resonators is increased by reducing the characteristic impedance of the resonator to 10 xcexa9 though the characteristic impedance of an input/output line section of that filter is set at 50 xcexa9.
However, when trying to suppress the current concentration, that is, the reduction of the current density, according to the above conventional example, since the line width is enlarged over the entire length of the lines forming the resonators by merely lowering the characteristic impedance of the lines, there is a problem that the filter formed by arranging these resonators in a line ends up becoming unavoidably large.
When applying the above prior art to a superconducting filter configured of a plurality of resonators obtained by bending xcex/2 resonators in a hair pin shape arranged in a line, being widely employed in recent years for the improvement of the power resistance, the superconducting filter becomes considerably large in size. If forming that superconducting filter on an inexpensive leading substrate (MgO, etc.) having a diameter of about 5 cm, just placing five resonators on that substrate becomes a handful. The problem then is that the intended steep cut characteristics can no longer be obtained.
In consideration of the above problems, an object of the present invention is to provide a superconducting microstrip filter capable of achieving an improvement of the power resistance while making it possible to maintain a current density below the critical current density (Jc) without making the overall filter large in size.
In further detail, another object of the present invention is to provide a configuration effective as a filter for reception waves and a configuration effective as a filter for transmission waves. Here, according to the above example, a xe2x80x9cfilter for reception wavesxe2x80x9d means a filter effective particularly with respect to the input power received by the receiver apparatus of the base station from the subscriber side, while a xe2x80x9cfilter for transmission wavesxe2x80x9d means a filter effective particularly with respect to the wraparound power due to the transmission power output by a transmitter apparatus paired with that receiver apparatus at a close distance at that base station or with respect to the transmission power directly received from another antenna of that base station. Note that the frequency band is different between the reception waves and the transmission waves.
Still another object of the present invention is to provide a superconducting filter which can be applied as a filter for reception waves, as a filter for transmission waves, or as a filter for both of the reception waves and transmission waves.
To attain the above objects, the present invention proposes the following first to fifth aspects:
A first aspect is a superconducting microstrip filter having a resonator section including at least one resonator, wherein the resonator forms a current density reduction part in one part of a line pattern thereof. This is a filter for reception waves.
A second aspect is a superconducting microstrip filter having a resonator section including a plurality of resonators cascaded in a line along a propagation path of signals to be filtered, wherein at least the resonators cascaded at the center portion of the propagation path and in the vicinity thereof form current density reduction parts in parts of the line patterns thereof and form the current density reduction parts larger in the resonators nearer the center portion. This is also a filter for reception waves.
A third aspect is a superconducting microstrip filter having a resonator section including a plurality of resonators cascaded in a line along a propagation path of signals to be filtered, wherein at least resonators cascaded at the center portion of the propagation path and in the vicinity thereof form current density reduction parts over the entire lengths of the line patterns thereof and form the current density reduction parts larger in the resonators nearer the center portion. This is also a filter for reception waves.
A fourth aspect is a superconducting microstrip filter having an input line section to which signals to be filtered are input and a resonator section arranged adjoining this input line section and including at least one resonator, wherein that input line section forms a current density reduction part in one part of its line pattern. This is a filter for transmission waves.
A fifth aspect is a superconducting microstrip filter having an input line section to which signals to be filtered are input and a resonator section arranged adjoining this input line section and including at least one resonator, wherein only that input line section is formed by a line pattern made of a material other than a superconducting material. This is also a filter for transmission waves.
The first to fifth aspects can be realized separately and independently from each other and also can be realized as a combination of some aspects. This will be clarified by the following explanation.